The invention relates generally to the art of large diameter composite gears and more specifically to a gear comprised of a carburized gear ring and a separately formed hub and a method of making such a gear so as to eliminate warpage or distortions, particularly in the gear teeth, resulting from the carburizing of the gear ring.
In the art of making large gears, it is common to make the gear hub and gear ring separately which are later joined to complete the gear. It is also common to carburize or case harden at least the toothed portion of the gear ring in order to reduce wear on the gear teeth during use.
The term carburizing as used in this specification will be understood to further include the process of case hardening and other similar hardening processes.
The desirable result of carburization is the formation of an outer shell on the treated part which has a higher carbon content, as much as 0.90% or more carbon at the surface, than the underlying, low-carbon steel, generally less than 0.25% carbon, the carburized layer being harder and, thus, more wear resistant than the original part.
Carburization of gears usually causes a warpage of the parts treated which results in the development of a concavity or cupping of the outer edge surfaces of the gear teeth, when viewed in circumferential cross-section which concavity must be removed by machining. The gear ring may also be warped out of round and/or out of its major disk plane so that it may no longer be flat or circular, this being especially true in large gears since quenching presses for gears larger than 36 inches in diameter are unavailable.
Carburized layers are generally only about 0.010 to 0.200 in. thick. Thus, in (0.250 to 5.0 mm) the machining of those portions of the gear which are out of plane and/or machining the gear teeth so the machined surfaces are again true, any substantial machining to eliminate the warpage effects may also reduce or eliminate portions of the carburized layer altogether, thereby losing some or all of its strength and wear-retarding effectiveness. Also, machining the carburized layer is very difficult and time consuming because of the hardness of the metal. In machining a large diameter gear, as much as 250 hours of machining on large and expensive equipment are required to eliminate distortions caused by carburization.